The present invention relates to a vacuum glow-discharging treatment method and an apparatus therefor, in which opposite surfaces of a web such as a plastic film or a metallic sheet is subjected to vacuum glow-discharging treatment in a vacuum condition in order to improve adhesive property, hydrophilic capability and dyeing capability of the web against another web or a coating material.
In the case that a photosensitive material is produced, it is well known in the art that opposite surfaces of a web-like support made of a plastic film are activated before the support is coated with a coating liquid such as an emulsion, in order to improve adhesiveness of the coating liquid to the support, and hydrophilic capability and dyeing capability of the support. In order to activate the opposite surfaces of the support, a method wherein opposite surfaces of a web-like support are subjected to glow-discharging treatment by applying a high voltage to a plurality of electrodes arranged so as to face the opposite surfaces of the support, is practically used (see U.S. Pat. No. 3,288,638, Japanese Patent Laid-Open Publication Nos. 53-13672, 54-80373, 56-1337, 56-1339 and 5-19899).
The method is practiced, for instance, such that a support is placed between a pair of metallic electrodes under a vacuum condition (e.g., 0.01 to 20 Torr) while applying a high voltage of 300 to 3000 V to the pair of electrodes with flowing electric current density of 0.5 to 50 mA per a unit area of each electrode. Japanese Patent Laid-Open Publication No. 53-13672 discloses an apparatus for activating the opposite surface of a web-like support employing such glow-discharging treatment, wherein a plurality of rod-shaped electrodes extending in parallel with each other in the transverse direction relative to the web-like support are arranged to face the opposite surfaces of the web-like support. One side of each rod-shaped electrode facing the support is constituted by a curved smooth surface, and a plurality of rod-shaped electrodes each designed in that way are arranged in the spaced relationship with a predetermined gap held between adjacent ones. In this apparatus, a distance between the support and the rod-shaped electrodes can easily be controlled by properly controlling a conveyance path of the support, so that surface treatment can satisfactorily be conducted with a desired degree of surface improvement.
When a long web-like support is continuously subjected to glow-discharging treatment, the temperature of each electrode is raised as electricity is discharged from the respective electrodes. On completion of the continuous electricity discharging for two to three hours, the temperature of each of the electrodes is elevated to a level of about 150.degree. C. to 200.degree. C. Each electrode is usually made of a metallic material such as aluminum, stainless steel or the like, and as the temperature of the electrode is raised to the foregoing level, the electrode may be thermally expanded and deformed. Especially, in the case that each electrode is prepared in the form of a rod-shaped electrode as mentioned above, the electrode of which opposite ends are supported by some means may be deflected due to the thermal expansion. Consequently, the support conveyed above or below the electrode thus deflected cannot be subjected to uniform surface treatment in the transverse direction of the support since the distance between the electrode and the support in the transverse direction is not constant.
Further, there is an adverse possibility that metallic components in the vicinity of the electrode may be brought in contact with the electrode, causing the components to be undesirably damaged. This damage on the components also leads to the result that an air-tight property for a vacuum condition cannot be maintained.
Furthermore, when the temperature of each electrode is raised, the sheath layer of a cable electrically connected to the electrode is thermally deteriorated, to shorten the running life of the cable. If the sheath layer of the cable is excessively deteriorated, a part of the sheath layer can crack, causing abnormal electric discharge from the cracked part of the sheath layer.
Therefore, with the conventional apparatus, since it is necessary to avoid excessive elevation of the temperature of the electrode when electricity is discharged from each electrode, continuous electricity discharging treatment cannot stably be conducted for a relatively long time period, i.e., more than two hours.
In addition, since the electrode is likely to be worn or degraded by spattering or the electrode and associated components must be replaced because of the foregoing unexpected problems, the electricity discharging apparatus is generally required to have a simple, replaceable arrangement to allow completion of replacing operations in minimal time.